1. Field of the Invention
The present invention relates to a detector detecting an advance state of a plasma etching process, particularly an endpoint of the plasma etching process.
2. Description of the Related Art
Generally, as shown in FIG. 3, a plasma etching device comprises a vacuum process chamber 1, serving as an etching chamber, and a pair of parallel plate electrodes 2 and 3, which are provided in upper and lower portions of the vacuum chamber 1 to have a predetermined distance. A gas introducing tube 4 and and a gas discharging tube 5 are connected to the vacuum process chamber 1. The gas introducing tube 4 is used to introduce CF series etching gas such as CF.sub.4 into the vacuum chamber 1. The gas discharging tube 5 is used to discharge gas generated in the vacuum chamber 1 to an outer unit of the vacuum chamber 1. Among the pair of electrodes 2 and 3 provided in the vacuum chamber 1, the lower electrode 2 is formed on a bottom surface of the vacuum chamber 1 to be used as a base for mounting a processing object, such as a semiconductor wafer W. The lower electrode 2 is a ground electrode, and the upper electrode 3 is connected to a high frequency power source 6. Due to this, if a high frequency voltage is applied to the upper electrode 3 to be discharged between the upper and lower electrode 3 and 2, plasma P due to etching gas, which is introduce to the vacuum process chamber 1, is generated, and, for example, the semiconductor wafer W, which is mounted on the lower electrode 2, is etched by the activated species of etching gas, which is present in plasma P.
In order to realize the improvement of etching accuracy and automatic etching, an emission spectrum of plasma P due to etching gas is guided to an endpoint detector A for detecting an endpoint of a plasma etching process through a view window 1a formed in the vacuum process chamber 1. The endpoint detector A comprises a photo detector 7 and a calculating section 8. The photo detector 7 sequentially detects the emission spectrum of the plasma P emitted from the view window 1a of the vacuum process chamber 1 to be photoelectrically transferred. The calculating section 8 calculates an advance state of the etching based on a detection signal of the photo detector 7. An electrical signal sent from the calculating section 8 is sent to a controller 9, and the controller 9 controls the drive of the high frequency power source 6 based on the electrical signal sent from the calculating section 8. According to the above-mentioned structure, the etching process, which is suitable for a predetermined pattern, is provided to a surface of the semiconductor wafer W until the endpoint of the etching is detected by the endpoint detector A.
As shown in FIG. 4, the photo detector 7 comprises an optical system, which comprises an aberration corrected lens 71, an incident slit 72, a diaphragm 73, a concave surface diffraction grating 75, a first light-receiving sensor 79, and a second light-receiving sensor 82. The aberration corrected lens 71 converges the emission spectrum of the plasma P emitted from the view window 1a of the vacuum process chamber 1. The incident slit 72 is provided at a focal point of the aberration corrected lens 71. The diaphragm 73 reduces the emission spectrum passed through the incident slit 72. The concave surface diffraction grating 75 receives the emission spectrum sent from the diaphragm 73 through a reflection mirror 74. The first light-receiving sensor 79 comprises a photodiode for receiving a first-order diffracted light having a specific wavelength sent from the concave-surface diffraction grating 75 through reflection mirrors 76, 77 and an emission slit 78 so as to be photoelectrically transferred. The second light-receiving sensor 82 comprises a photodiode for receiving a zero-order diffracted light sent from the concave surface diffraction grating 75 through a reflection mirror 80 and an interference filter 81 so as to be photoelectrically transferred.
According to the above-mentioned optical system, the photo detector 7 can detect a specific wavelength of the emission spectrum of plasma P. Generally, the wavelength to be detected by the photo detector 7 is determined by kinds of etching gas. In a case where a silicon oxide film is etched by use of etching gas of CF series such as a CF.sub.4, a specific wavelength of the emission spectrum of an activated species CO* which is a reaction product, such as 219 nm or 483.5 nm is detected by the photo detector 7. Also, In a case where a silicon nitride film is etched by use of etching gas of CF series such as a CF.sub.4, a specific wavelength of the emission spectrum of an activated species N* which is a reaction product, such as 674 nm is detected by the photo detector 7.
More specifically, in the case where a silicon oxide film is etched, an light-receiving angle of the concave-surface diffraction grating 75 is set to have a predetermined angle in order that the first-order diffracted light whose wavelength is, for example, 219 nm is guided to the first light-receiving sensor 79 from the concave-surface diffraction grating 75 through the reflection mirrors 76, 77 and the emission slit 78. At the same time, in order to that the zero-order diffracted light from the concave-surface diffraction grating 75 having the above-set light-receiving angle incident onto the second light-receiving sensor 82, in other words, light, which changes in accordance with density of the emission spectrum of the vacuum process chamber 1, is made incident onto the second light-receiving sensor 82, the optical system, which comprises the reflection mirror 80, the interference filter 81 and the second light-receiving sensor 82, is set to have a predetermined light-receiving angle. Then, in the wavelength detection under the above-mentioned state, the calculating section 8 corrects an electrical signal s.sub.1, which is based on the first-order diffracted light, sent from the first light receiving sensor 79, by an electrical signal s.sub.2, which is based on the zero-order diffracted light, sent from the second light receiving sensor 82 to smooth a drift phenomenon of light having the specific wavelength, which is to be detected, thereby the change of light having the specific wavelength is correctly captured, and the endpoint of etching is correctly calculated.
In the above-structured endpoint detector A, if the object to be etched is changed from the silicon oxide film to the silicon nitride film, the specific wavelength of the first-order diffracted light to be extracted through the concave-surface diffraction grating 75 is changed from 219 nm to 674 nm. Due to this, it is needed that the concave-surface diffraction grating 75 be rotated to change the light-receiving angle. However, in the optical system of the above-mentioned conventional endpoint detector A, the zero-order diffracted light sent from the concave-surface diffraction grating 75 is detected in order to smooth the drift phenomenon of light having the specific wavelength which is to be detected. Due to this, if the light-receiving angle of the concave-surface diffraction grating 75 is changed, an optical path of the zero-order diffracted light is varied. Due to this, there must be also changed the light-receiving angle of the optical system including the reflection mirror 80 for capturing the zero-order diffracted light from the concave-surface diffraction grating 75, the interference filter 81, and the second light-receiving sensor 82. Due to this, there is a problem in that the adjusting operation is extremely complicated.